Field of the Invention
The present invention relates to halftone processing.
Description of the Related Art
Image processing apparatuses in which an image represented by image data is reproduced using a digital printer such as a laser beam printer have become widespread. To reproduce the halftone of an image, a method of performing tone reproduction by halftone processing is employed generally.
Dot concentrate type halftone processing (AM screen processing) using an ordered dither method is processing of deciding an output value by comparison with a threshold which varies periodically. The AM screen processing can provide a satisfactory halftone processing result with respect to an electrophotographic process because halftone dots of the same shape are formed at equal intervals at the flat portion of an image.
However, in AM screen processing for an image having a periodic pattern, like halftone dots, or a character or fine line portion, the period of the AM screen (period of the threshold) and a high-frequency component contained in an image interfere with each other. Especially when a periodic pattern close to the period of the AM screen is input, a strong interference occurs and a periodic fringe called moiré appears.
In AM screen processing, the tone is expressed by the dot area, so the spatial resolution expressible by halftone decreases. The edge portion of an image is expressed by dots arranged stepwise, causing degradation of the image quality, called “jaggy.” On a low-density fine line, the density may fluctuate depending on the position of the line, or part of the fine line may be lost.
Japanese Patent Laid-Open No. 2010-074627 (literature 1) discloses a method of processing an image by using a plurality of different screens, deciding, based on the feature analysis result of the image, the composition ratio of processing results using the respective screens, and outputting an image having undergone composition processing.
Japanese Patent Laid-Open No. 2010-245591 (literature 2) discloses a method of determining the edge portion and non edge portion of an image, and processing the edge portion and non edge portion using different screens. Literature 2 also discloses an edge determination method of determining that a pixel adjacent to a line (“negative line” in literature 2) of a lower density than the background density is a non edge portion.
The methods disclosed in literatures 1 and 2 require a processing measurement for performing a plurality of different screen processes, and an analysis measurement for performing feature analysis of an image, thus increasing the circuit size of the halftone processing unit. Also, pixels at a mixing portion of different screen processes and a switching portion of screen processing stand out and may be recognized as degradation of the image quality.
Japanese Patent Laid-Open No. 2005-252583 (literature 3) discloses a method of combining an ordered dither method and error diffusion method (FM screen), and reducing the deviation of an error between a conversion value in dither processing and an output value. According to the method disclosed in literature 3, which of ordered dither processing and error diffusion processing is to be applied needs to be selected prior to halftone processing of each pixel. It is difficult to appropriately make this selection using a simple selection measurement.